PROJECT SUMMARY/ABSTRACT Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially life-saving treatment for refractory or relapsing hematological malignancies such as acute myeloid leukemia or multiple myeloma. The antitumor effects of HSCT appear to be due to the activation and differentiation of allogeneic (immune mismatched) donor T cells that recognize and eliminate tumor cells via a mechanism called graft vs. tumor. Although HSCT has been shown to be more effective than chemotherapy for treating these aggressive malignancies, approximately 35-50% of patients undergoing allogeneic HSCT will develop multi-organ, inflammatory tissue injury called acute graft vs. host disease (aGVHD). The precise immuno-pathogenic mechanisms responsible for aGVHD have not been definitively defined; however, the vast majority of preclinical and clinical studies suggest that the onset and severity of this disease is potentiated by the tissue-damaging effects of pre- transplant conditioning protocols such as total body irradiation and/or chemotherapy. This initial damage to the gut as well as other target tissues (e.g. skin, liver and lungs) promotes the generation of pro-inflammatory cytokines and mediators as well as translocation of intestinal bacteria into the gut tissue. Both of these events are thought to contribute to the activation and expansion of donor-derived, allogeneic CD4+ and CD8+ T cells that mediate inflammatory tissue injury in the lungs, liver, skin and gut. However, recent preliminary studies from our laboratory, suggest that intestinal injury may not be required for the development of aGVHD. Our preliminary studies demonstrate that adoptive transfer of allogeneic CD4+ T cells into healthy/untreated lymphopenic recipients induces many of the clinical and histological features of aGVHD including weight loss and reduced activity as well skin, lung and liver inflammation. Based upon these findings, our overall objective is to better understand the role that T cells and the intestinal microbiota play in the pathogenesis of aGVHD in the absence of intestinal injury. We hypothesize that allogeneic CD4+ T cells are both necessary and sufficient to induce aGVHD in the absence of intestinal damage. In order to test this hypothesis, we propose the following three specific aims: 1) We will quantify and compare the onset and severity of tissue inflammation following adoptive transfer of allogeneic CD4+ and/or CD8+ T cells into healthy lymphopenic recipients; 2) We will define the role that the T cell-associated epigenetic modifier sirtuin 1 plays in the generation of disease-producing effector cells and 3) We will determine the role that the intestinal microbiota plays in the induction and progression of aGVHD in healthy lymphopenic mice. Data obtained from the proposed studies will advance our understanding of the immunological mechanisms responsible for induction of aGVHD in the absence of intestinal injury. In addition, these data may have important implications for newer clinical studies that are exploring the use of reduced intensity in pre-transplant conditioning to decrease severity of aGVHD.